Passive Matrix (PM) addressing is a technology used in early Liquid Crystal Displays (LCD). Passive matrix addressing uses one group of parallel conductors as column electrodes, and another group of parallel conductors as row electrodes to form a grid structure. Liquid crystal pixels are sandwiched between the column electrodes and the row electrodes. Each pixel is controlled by an intersection of the row and column electrodes. By applying a proper voltage at an intersection, the liquid crystal responds by creating an “on” state at that intersection. By using this scheme, only (m+n) control signals are required to address a display with n columns and m rows. The signal is divided into a row signal and a column signal. The row voltage determines the row being addressed and all n pixels on a row are addressed simultaneously. When pixels on a row are being addressed, a selection potential (Vsel) is applied to that row, and a non-selection potential (Vunsel) is applied to all other rows. The column potential is then applied with a potential for each column individually. In a display, an on (lit) pixel corresponds to a potential of Von, and an off pixel corresponds to a potential of Voff.
An example of a display using passive matrix addressing is illustrated in FIG. 1. Specifically, as shown in FIG. 1, a two-dimension array 100 has a plurality of rows and columns. A column is selected by applying a column signal as shown by arrow 110, and a row is selected by applying a row signal as shown by arrow 120. As a result of the column signal and the row column being applied, the pixel corresponding to the selected row and the selected column is turned on.
Passive matrix technology is a low cost solution and is easy to implement. However, problems arise as the number of rows and columns increase. With higher pixel density, the electrode size must be reduced and the amount of voltage necessary to drive the display rapidly increases. The higher driving voltage creates the secondary problem of cross-talk. Even though only one row and column are selected, liquid crystal material near the row and column being charged are affected by the pulse. The net result is that the selected pixel is active, and pixels surrounding the selected pixel are also partially active. The partially active pixels reduce the display contrast and degrade image quality.
Passive matrix addressing has also been used in Organic Light Emitting Diode (PMOLED) based displays, especially in small size displays. Although the concept of the PMOLED structure is relatively straightforward to design and fabricate, PMOLED displays also face cross-talk issues. As a result, PMOLED displays are normally most suited to display applications where the display size is less than about 50 millimeters to 80 millimeters across the diagonal or where there are less than about 100 rows. As analyzed by D. Braun in his paper “Crosstalk in passive matrix polymer LED displays”, Synthetic Metals (1998) 92, p. 107-113, there are serious cross-talk issues in a passive matrix structure.
Reference is now made to FIG. 2A, in which a passive matrix array of LEDs is shown. In FIG. 2A, rows 1 through 4 form the cathode electrodes, and columns A through D form the anode electrodes. As current is applied through LED A1, some of that current may also flow through series of LEDs A2, B2, and B1; through LEDs A2, C2, and C1; through LEDs A4, B4, and B1, amongst other combinations, as illustrated in FIG. 2B. In each case, two LEDs are forward biased and one is reverse biased. With LEDs acting like ideal diodes, the reverse-based LEDs would not permit any current flow, and no light could emerge from the forward-biased LEDs. However real LEDs do permit some current to flow under reverse bias, and several reverse-biased LEDs in parallel can pass enough current to allow light emission from another LED in series. For example, pixel A2 lies in series with pixels B2, C2, and D2. Even a low-information-content display would have more than 4×4 pixels, and therefore more parallel conductions paths typically exist.
According to Braun, ibid, there are several potential sources of direct current (DC) cross-talk in LED matrix displays.                Display resolution: the greater number of pixels required for higher-resolution displays creates more available parallel conduction paths, so one would predict more stray light in higher-resolution displays.        Rectification ratio: one rough way to characterize diode quality is in terms of the ratio of current flowing in forward bias to that flowing under the same magnitude reverse bias. A higher rectification ratio indicates that less leakage current flows for a given magnitude bias, so less stray light should occur with higher-rectification devices.        Reverse leakage current: another way to characterize the reverse-bias quality of a diode is in terms of the parallel leakage paths that compromise the diode action under reverse bias. Larger reverse leakage current would increase the stray light generated.        
The passive matrix addressing scheme has also been applied to photodetector arrays due to its simple design and ease of fabrication. The issue of cross-talk however gets even worse in PM photodetector arrays because of the orders of magnitude lower rectifying ratio of the photo detectors under illumination. FIG. 3 shows current-voltage characteristics of a typical organic solar cell in the dark and under irradiation, and illustrates a worsened rectifying ratio under light.
Therefore, serious cross-talk issues exist in passive matrix photodetector arrays. U.S. Pat. No. 6,303,943 to Yu et al. proposes to use switchable photosensitivity of specially designed photodiodes to achieve a switching effect in an attempt to overcome cross-talk issues in a passive photodetector array, where the photosensitivity can be switched on and off by the biasing voltage across the detectors, where the switching voltage imparts photosensitivity above 1 mA/W at a preselected operating bias and near zero photosensitivity at a cut-off bias substantially equivalent in magnitude to the built-in potential of the photodetector. The photocurrent can be probed with a read-out circuit in the loop. These photodetectors can be arranged in linear arrays or in two-dimensional matrices that function as linear or two-dimensional image sensors. “Simulations of Passive Matrix Polymer Image Sensors”, by D. Braun and G. Yu, MRS Proceedings, 1999, 558, propose a similar system.
A more commonly used solution to overcome the cross-talk issue is to use an active matrix arrangement where arrays of transistors are connected to pixels, functioning as switches to turn on and off the pixel current path. However, this approach is very expensive.
Cross-talk in passive matrix addressing can also be controlled by using an array of diodes as switchers by controlling the bias direction and threshold voltage. For example, in a photodetector array, each photodetector will connect to a non-photosensitive switching diode in a head-to-head or tail-to-tail fashion. This arrangement is relatively easy to implement. Similar structures were used by others in some x-ray detectors and image sensors using traditional semiconductor materials such as in U.S. Pat. No. 4,604,527 to Chenevas-Paule et al., U.S. Pat. No. 5,229,858 to Ikeda et al., and U.S. Pat. No. 5,523,554 to Hassler et al.
Reference is now made to FIG. 4A and FIG. 4B, which illustrate a scheme whereby a switching diode is used to control when current can flow. In FIG. 4A, a negative voltage Voff is applied to the circuit and the switching diode 410 is turned off. As a result of the switching diode being turned off, no current may flow through photo diode 420. In FIG. 4B, a positive voltage Von is applied to the circuit and the switching diode 410 is turned on. As a result of the switching diode being turned on, current may flow through photo diode 420.
The above described methods are relatively easy to implement in the Si-microelectronics industry, due to the high precision of fabrication processes, well defined layer thickness, and extremely low variations among produced devices in Si electronics. However, the same device arrangement is not straightforward to implement in the printable electronics industry, where the layer-to-layer alignment accuracy is low, printing resolution is limited to about 100 micrometers, printed layers are subject to large thickness variation, and where there are large performance variation and low yield.